Dozens of scientists, artists, writers and environmentalists on Friday urged the leaders of Mexico, Canada and the United States to devote part of their meeting next week to discussing ways to protect the monarch butterfly.

A letter to the three leaders signed by more than 150 intellectuals, including Nobel literature laureate Orham Pamuk, US environmentalist Robert Kennedy Jr and Canadian author Margaret Atwood, notes the monarch population has dropped to the lowest level since record-keeping began in 1993.

Mexican President Enrique Pena Nieto, US President Barack Obama and Canadian Prime Minister Stephen Harper Obama are meeting in Toluca, near Mexico City, on Wednesday to discuss such matters as economic competitiveness, trade and investment, entrepreneurship and security.

The monarch's spectacular annual migration to spend the winter in Mexico is little understood. Experts blame the drop in numbers on several things: extreme weather trends, a dramatic reduction of the butterflies' habitat in Mexico from illegal logging, and genetically modified crops in the US displacing milkweed, which the species feeds on.

The petition, put together by Mexican writer and environmentalist Homero Aridjis, says Mexico is addressing the logging problem and calls on the US and Canada to deal with the impact of their agricultural policies.

After steep and steady declines in the previous three years, the black-and-orange butterflies now cover only 1.65 acres (0.67 hectares) in the pine and fir forests west of Mexico City, according to a report last month by the World Wildlife Fund, Mexico's Environment Department and the Natural Protected Areas Commission.

Monarchs covered more than 18 hectares at their recorded peak in 1996.

Because the butterflies clump together by the thousands in trees, they are counted by the area they cover.